Noise suppression circuit



G. J. EANNARINO NOISE SUPPRESSION CIRCUIT Filed DeC. 6, 1946 April 3, 1951 Patented Apr. 3, 1951 UNITE STATES TENT QFFECE (Cl. 25d-6) 5 Claims. i

The present invention relates to improved radio control systems and, more particularly, to remote control systems wherein radio receivers, which are controlled in operation by transmitted radio signals, are protected from improper operation.

Communication systems in which transmissions from a main station must be directed to selected receivers o a plurality of associated receivers are well known and are widely employed, for example, in police systems wherein communications must be maintained between a central location and a plurality of mobile units.

It may be found in communication systems of this type that interfering carriers may cause undesired receiver operation and that certain receiver noise signals may also render the receiver operative by simulating the reception of the characteristic audio tones. These phenomena are obviously undesirable when it is preferred that the receivers respond only to a certain carrier or to tones intentionally transmitted thereto.

It is therefore one of the major objects of the present invention to provide a. radio control system wherein the above difliculties are avoided,

and another main object is to provide, for use in such systems, radio receivers wherein noise signals are utilized to prevent improper receiver operation,

With these and other objects in View, the present invention will be explained in the following detailed description which is to be read in con- `iuncticn with the block and schematic diagrams shown in the accompanying drawing.

The radio control system as an integrated unit f comprises at least one transmitting system and preferably a plurality of receivers, 2 and 3 being representative. Transmitting system I is adapted to radiate at least one carrier, and this carrier may be either amplitude or frequency modulated, depending upon preference and system requirements, by the desired intelligence signals. Additionally, the transmitter 4 may be modulated by any one or a combination of a plurality of audio signals from the source of reed-frequency signals 5, these signals being selectively coupled to the transmitter, for purposes which will later become obvious, by means of switches such as 6 and 'I.

A typical receiver arrangement suitable for use in the system of this invention is disclosed in concludes the conventional receiver lineup. Muting control of the receiver output is achieved by switching system I2 shown coupled to the audio ampliers Ill. Excitation for the switching system I2 may be derived from the high frequency stages 8 and also from a tuned reed system I3. This reed system receives its actuating signals from the audio ampliers I0. In addition, the noise circuit I4 actuated by output from ampliers Ii! delivers its output signals to switching system i2.

Fundamental operation ofthe improved control system may be outlined as follows with respect to the preceding disclosure. Transmitting system I radiates intelligence signals modulated upon a certain carrier, which carrier may in turn be picked up by receiver 2, amplified in stages 8 and detected by 9, the audio components being amplied and reproduced by amplifiers IIl and speaker II. Switching system I2 is adaptable to prevent transmission of signals to the speaker from ampliers lil except when said switching systems is actuated by certain voltages from stages 8 and reed system I3. Noise circuit i4 provides a bias for switching system i2 which is of one polarity which insures that system I2 will iute the receiver output as long as noise signals appear in audio amplifier I0 and as long as the biasing voltages of a second polarity delivered to system I2 by stages 8 and reed system I3 do not equal or exceed the voltage output of noise circuit Il. If it is desired that the receiver be made operable to deliver an audible output simply upon reception of a desired carrier, switch I5 should be closed, thus coupling stages 8 with the switching system I2 and supplying the system with actuating voltages when the desired carrier is received. During the intervals when a carrier is being received, negligible noise is delivered to noise circuit Id. from amplifiers I0, and the bias on switching system I2 due to the output from circuit I4 substantially ceases to exist, thereby permitting the actuating voltages from stages 8 to actuate system I2 through switch I5 and to render the receiver operative. These actuating voltages, which are delivered by stages 8 only when the desired carrier is received, also appear in coupling line I6 between stages 8 and system l2, although the switching system I2 is adapted to be actuated by these voltages only when such voltages have first been received either from stages 8 through switch I5, or, as later described, from reed system I3. It should therefore be apparent that when the receiver is carrier controlled, as above described, output signals may be delivered only when the actuating voltages from stages 8, which are related to signal strength of the desired carrier, exceed the voltage output of noise circuit l, which is proportional to the noise appearing in amplifiers lil. When interfering car- 'iers of low signal strength are received the output of noise circuit Ill is usually of sufficient magnitude to prevent actuating of switching system i2 by the low amplitude actuating voltage delivered to the system from stages 8.

lf2, however, it is preferred to r the receiver operable only When a certain tone modulates the transmitter carrier, switch l5 is left open. Transmitter i may be modulated by said certain tone from the source of reed-frequency signals 5 when one or a combination of switches suoli as and 'i are closed. At receiver 2., said certain tone will be delivered to tuned reed system i3 by ampliiiers iD, and system i3 will thereupon transmit an actuating bias of said secnder ond polarity to the input of switching system i2. This actuating voltage vf l cause switching system l2 to operate the receiver providing that no appreciable bias or the opposite polarity established on the switching system by the noise circuit Irl, a condition which exists when a carrier modulated by said certain tone is being received. If certain noise signals or transient conditions in the receiver cause the tuned reed system i3 to be actuated when said certain tone is not received, the switching system l2 will not render the receiver operative, since the bias produced by noise circuit ld in the absence of a received carrier is adjusted to exceed the opposite polarity actuating bias output of tuned reed system I3. However, once the carrier modulated by said certain tone is received and said actuating voltage is delivered to the then substantially unbiased switching system l2, the biasing voltage of said second polarity from stages 3 delivered by coupling lines ill will maintain actuation of the eystern i2 while the desired carrier is received, even though Said tone no longer modulated on the carrier. Cessation of the carrier operates to stop actuation of switching system l2 by biasing voltage of said second polarity and the receiver thereupon becomes inoperative until the tone is again received. Noise circuit lll, responsive to signals occurring when the carrier is no longer received, biases the switching system iwith a voltage of said one polarity to insure that the receiver is maintained inoperative.

The diagram the receiver designated by numeral. :l illustrates in greater detail an embodiment of the present invention. This receiver, which may be identical with the receiver '2, is designed for reception of frequency modulated signals, and, included in the receiver lineup are R.. F. arnplier ll, local oscillator i6, mixer I9, l. F. amplifier 2li, rst limiter stage El, the second limiter 22, discriniinator 23, audio amplifier stage 21%, power amplifier stage 25, and a speaker 253. That portion-of receiver 3 which corresponds to theswitching system l2 of receiver 2 includes .primarily the double triode 2l and the multiple contact relay 28; and the tuned reed system i3 of receiver 2 has as its counterpart receiver 3 the tuned reed switching arrangement 2e utilized in conjunction with the biasing battery 35i.

The receiving set 3 can, as previously described with respect to receiver 2, be used either for carrier controlled operation or for selective calling operation wherein a selective signal cmi ted by the transmitter controls the reproduction of in telligence signals by the receiver.

Referring now to the carrier controlled opera tion of receiver 3, one portion of the control cir cuit includes aainplier, as a tried-e 3 l, and a relay control tube, as triodc with he associated relay 28. Both triodes Si and 32 are preif-:rably in a single envelope. when the circuit is to be adapted for carrier controlled operation. switch 33 is closed, thereby coupling the negative grid voltage of the limiter tube 5K: to ground through resista ce 35 and normally closed contacts of the relay 2S. The negative voltage established across resistance is coupled to the grid of the ampliiier tube 3l prevents flow of plate current therethrough. When tube 3i is non-conducting, there is only a small volt age drop across the load resistance and the voltage appearing at ju:l ction of plate resistance 35 and grid resistance s is of a sufficiently positive value to per t tried-e 32 to conduct. The winding 3S oi multiple contact relay Z8 is thereby energized and plate voltage is applied to the power amplifier through armature 39 and contact ci relay 3 to render the receiver operative and capable reproducing audio signals.

The foregoing operation relates to a condition in which desired carrier signals are received. However, when no carrier is picked up the receiver 3, the triode 3l roust ce biased positively to prevent energizing of winding Sil of the multiple contact relay Ed and subsequent energizing of the audio power output st ge 2% and operation i the receiver. Noise circuit i accomplishes this by producing a positive IIL-C. responsive to the noise signals which always occur in the audio amplifier stage 2d when no carrier is received. This noise circuit includes a tuned circuit 42 in parallel with the series combination of the rectifier-filter circuit and resistances @ifi and 35, the circuit preferably being tuned to noise frequencies above the audio range, about or l5 kilocycles, which are impressed thereacross from the plate circuit of audio amplifier stage 2li. The noise signals froci circuit 2, when rectiiied and filtered by a circuit 43., produce a voltage drop across resistance 35 and lil which is opposite polarity to the voltage drop across resistance 35 resulting from the negative voltage applied thereto the grid circuit of first limiter tube 35i. The voltage applied to the grid 3| is tapped from across resistance [it and, when the receiver is carrier controlled, there .vill be a net D..C. voltage prod'- led by combina tion or" the negative voltage across 3E due to the grldvoltage of limiter tube 3Q and the positive voltage across .resistances 35 and a portion of resistance due to the rectiiled and filtered noise voltage impressed across series co1nbination resistance 35 and d4. Tube Si is conducting, and hence relay 28 is unoperated when the grid voltage of this tube is at approximately zero or positive bias value. The tap across resistance 41's is therefore adjusted such that the rectlned and ltered noise signals will produce a positive bias which at least equals or exceeds the negative bias reduced across resistance 35 when a carrier is not received. Upon reception of tne carrier by receiver -3 the positive bias on tu 3l due to noise signals will be negligible in comparison with the negative voltage produced across resistance 35, `tube 3i will be rendered non-conducting, relay winding 3S will be energized, armature 39 will close with contact 0, plate voltage Will be applied to the audio amplifier stage 35 and the receiver Will be rendered operative.

" -When the circuit of receiver 3 is to be adapted for selective calling operation, .the switch 33 is left open. Ience, when no carrier is received. the only voltage applied to the grid of tube 3i is the positive voltage tapped from across resistance 4f and resulting from the rectication and filtering of noise signals by noise circuit l. The audio power amplifier stage 35 therefore remains de-energized and the receiver is inoperative. When a carrier is received the grid voltage of the first limiter tube 3e will increase in a negative direction; however, since switch 3S is open, there is no bias established for the grid of triode Si by the voltage in the grid circuit of tube 34 and the relay 28 will not operate.

If then an audio tone of the frequency to which the vibrating reed unit 29 will respond is impressed on the carrier, this audio tone will be applied from the plate circuit of audio arnplier tube 24 to the reed winding E through Contact 46 and armature 3! of relay 2S. Contact il? and armature 8 will close when winding i5 is thus energized, and negative voltage from source 30 will be applied to the grid of D.-C. amplifier' triode 3l. Since a carrier is being received at this time, no positive bias will be produced on the grid of tube 3E by noise circuit ii and the voltage from source 3D will render tube El non-conducting, tube 32 conducting, relay winding S8 energized, armature 39 and contact il of relay 28 closed, ampliiier stage energized, and the receiver operative` Once the relay winding 3S has been thus actuated, armature 65 and contacts 58 of relay 28 will close, and the negative voltage from the grid circuit ofrlimiter tube Sli will be applied to the grid of triode Si. *hereby maintaining the receiver in an operative condition. Cessation of the carrier thereafter will reduce the negative voltage delivered to triode 3l from the lgrid of limiter tube and will result in a positive voltage being applied tc triode Si from the noise circuit 4l due to noise signals occurring in audio ampliiier stage 2.5 in the absence of a received carrier. The net positive voltage which is therefore applied to the grid of triode Si renders this tube conducting, triode 32 nonconducting, winding 3S unenergized, audio anipliiicr stage 25 unenergized, and the receiver inoperative.

'Ihe receiver 3 is thus protected against iniproper operation due to closing of armature 8 and contact il of tuned reed system 29 when no carrier is received. Suchreed actuation for example, occur as a result of certain noise signal' which are present in the absence of a received carrier. This protection is achieved by utilizing a voltage source 39 whose negative voltage output to the grid of triode 3l does not exceed the positive bias normally delivered to the triode Si from across resistance 44 and from the noise circuit M when no carrier is being received. Consequently, undesired closing of the tuned reed contacts will not cause operation of the receiver. v-

The sensitivity of this receiver system may be adjusted by varying the position of the ap on resistor 44. If the tap is moved in a direction which increases the resistances between the grid of tube 3l and ground, the4 positive bias on the triode 3l due to noise signals will be increased. Therefore, a greater negative voltage in the grid circuit of the first limiter tube 34 is required to cause operation of the relai.r 28 when the system is set for carrier controlled operation. The grid voltage of the first limiter tube 34 is a function 0f signal strength and up toa certain point in creases with the signal strength. f, for example, it is desired that the receiver shall not be rendered operative upon reception of low signal strength interfering carriers, the tap on resistance '44 should be moved to increase the positive grid bias on tube 3i due to noise signals from noise circuit d. Hence, when a lou1 signal strength interfering carrier is received the out put of the noise circuit may be of sui'icient magnitude to prevent actuating of relay 28 and subsequent operation of the receiver.

By utilizing the receiver noise signals to pro duce a bias cn the receiver meting arrangement, as herein described, it is possible, then, to prevent undesired operation o the receiver by said interfering carrier signals, to insure positive operation of the receiver when carrier controlled, and to prevent improper receiver operation resulting from actuation of the timed reed system by erroneous signals. y

While the principles of this invention have been disclosed indetail with respect to a mobile frequency modulation receiver, it should be clear that the saine principles can be used in connection with an amplitude modulation receiver by appropriate adaptations well known to those skilled in the art. The system as a whole has been digraninied with only a single transmitter and two receivers, although, of course, any number cf transmitters and receivers may be incorporated therein. Also, each receiver may be adapted to respond to a tone which is characteristic to its own reed alone, or several receivers may be adapted to respond to the saine tone. Any required number of tones may be derived from the source o reed signal frequencies 5 and these may be impressed simultaneously or individually upon the transmitter to achieve appropriate modulation of the carrier, depending upon whether i*L is desired to cause one or a plurality o receivers to respond to the transmitter signals. It may also be desirable to maintain operation of any receiver only'as long as a certain character istie audio tone is received, in which case this tone may be blocked from reaching the speaker and then, with certain obvious changes, the general arrangement diagramrned herein may function suitably.

There are many changes which may be instituted in the above circuits without departing either in principle or scope from the invention herein disclosed. For example, the rectiiier lter arrangement 43, which includes a vacuum tube in the accompanying drawing, may be satisfactorily replaced by a rectifier-lter which includes a dry rectier unit. Voltage source 30 associated with the tuned reed system 29 need not be a battery as shown and instead the required voltage may be tapped from any suitable point in the receiver circuit. Although it is preferred to tap the desired negative voltage, 'which occurs responsive to the reception of the carrier, from the grid circuit of the first 'limiter tube 34, it may be possible to obtain a suitable voltage from other points in the receiver. Also, additional contacts and armatures may be included in the multiple contact relay 28 to provide additional controls when this relay isv actuated. One such additional control being, for example, switching in the filament circuit in the audio power amplifier stage to vpermit a reduction in average filament current drain. It may also be preferred to interrupt portions of the audio power amplifier stage other than the plate circuit to i accomplish muting of the receiver output, in which case the obvious changes should be made.

While the principles of this invention have been disclosed in connection with preferred embodiments, it should be apparent from the foregoing discussion that these embodiments have been presented by way of example and preference only and should not be construed as limiting the'scope of the invention.

I claim:

l. A radio communication system, comprising: a transmitter, means for selectively modulating the transmitter carrier with any of a plurality of characteristic audio frequency tones, at least one receiver system adapted to receive a carrier radiated by said transmitter, including high frequency stages, detector means, audio amplifier and audio reproducing means, a muting device for controlling the audio output of said reeciver system, a, noise circuit for applying to said muting device a biasing voltage which is proportional in amplitude to certain noise signals in said receiver system and which is of one polarity suitable for actuating said muting device to mute the audio output of said receiver system, a tuned reed system, responsive to the occurrence of a certain characteristic audio frequency tone in said receiver system, for applying to said muting device a biasing voltage which is of a second polarity suitable for actuating said muting device to control said receiver system such that an audio output may be delivered thereby, and means for coupling a biasing voltage of said second polarity to said muting device, from a point in said high frequency stages where said biasing voltage exists responsive to the reception of a carrier, following actuation of said muting device by said tuned reed system.

2. A radio receiving system, comprising: means for receiving radio frequency carriers and for reproducing audio components thereof including high frequency stages, detector means, audio amplifiers and audio reproducing means, a multiple contact relay, an operating coil for said relay, a pair of normally open contacts connected between said audio amplifiers in the source ci energizing voltage for said amplifiers, a control device for energizing said relay operating coil responsive to certain biasing voltages, a noise circuit responsive to noise signals appearing in audio ampliers for producing a of polarity tending to prevent said controi device from energizing said operating coil a tuned reed system responsive to a characteristic audio frequency signaldemodulated by said receiving means for applying to said control device a biasing voltage of a second polarity tending to cause said device to energize said operating coil, and a second pair of normally open relay contacts connected between said control device and a point in said high frequency stages where biasing voltage of said second polarity appears when a carrier is received.

3. A radio receiving system according to claim 2 wherein said noise circuit comprises a resonant circuit coupled to a point in said audio ampliners where noise signals may appear and tuned to frequencies above the audible range, and a rectiiier-iilter combination connected to derive a D.-C. biasing voltage for said control device from across said resonant circuit.

4. A radio receiving system according to claim 2 wherein said control device comprises a first vacuum tube which conducts only when the net grid 1cias thereon, derived from said tuned reed system and Said noise circuit and said high frequency stages, exceeds a definite value, and a second vacuum tube connected in series with said operating coil and a power source and actuated by said first vacuum tube such that said second vacuum tube is rendered conducting when said first vacuum tube is non-conducting and said second vacuum tube is non-conducting when said rst vacuum tube conducts.

5. A radio receiving system comprising a frequency modulation receiving arrangement including high frequency stages, demodulating means, audio amplifier stages, and audio reproducing means, a multiple contact relay switching means, said relay switching means including an operating coil, a pair of normally open contacts of said switching means connected between said audio amplifiers and energizing means for said audio amplifiers, a vacuum tube to energize said operating coil, a tuned reed system responsive to a certain characteristic audio frequency tone in said audio amplifiers for applying to said vacuum tube a biasing voltage of one polarity such that said operating coil is energized, a noise circuit responsive to certain noise signals which may appear in said audio ampliers for applying to said vacuum tube a biasing voltage of a second polarity such that said operating coil is rendered unenergized, and a second pair of normally open relay switching means contacts connected between said vacuum tube and a point in said high frequency stages where a biasing voltage of said one polarity exists when a carrier is received.

GEORGE J. EANNARINO.

The foilowing references are of record in the le of this patent:

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